1. Field of the Invention
The present invention relates to magnetic metal powder and its manufacturing method.
2. Description of Related Art
The manufacturing method of metal powder can be classified by its starting raw material. In other words, metal powder can be manufactured from its gaseous phase, liquid phase and solid phase. And, as a specific method for manufacturing metal powder from the gaseous phase, the known methods are a chemical vapor deposition (CVD) method, sputtering method and vacuum deposition method. As for methods of manufacturing metal powder from the liquid phase, the known methods are a co-precipitation method, gas or water atomization method, spray method and spray pyrolysis method. As for making metal powder from solid phase, there is a pulverizing method that uses a crusher to pulverize metal nuggets into particles of appropriate sizes or administering a prescribed process on the pulverized powder.
Various parts used in the electronics field will be more frequently and widely used in the high frequency range. The same can be said about printed circuit boards. Substrates with various characteristics will be in demand such as those with high or low dielectric constant, high magnetic characteristics or those that absorb radio waves. To obtain these substrates, magnetic powder with excellent high frequency characteristics are being mixed and dispersed into printed circuit boards according to its needs. Some of the magnetic powders being used are ferrite powder and carbonyl iron powder for high frequency use. In areas other than printed circuit boards, there is the packaging category where radio wave absorbing powders are mixed and dispersed within resin. In the field of conductive pastes, conductive particles are mixed and dispersed in thick film pastes to manufacture electronic circuits, resistors, capacitors and IC packages. Moreover, in soft magnetic materials, magnetic powder is used widely for making coil materials for power supplies like choking coils. As for magnetic materials, there are core materials for motors. Magnetic powder is also used in magnetic resistors and magnetic sensors.
A technique for creating metal powder for thick film paste using the spray pyrolysis method is known. This technique entails spraying a solution containing metal salts to create liquid droplets, and heating the droplets at a temperature higher than the metal salt decomposition temperature and at a temperature higher than the metal melting point, but if the metal forms an oxide at temperature below its melting point, at a temperature higher than the oxide decomposition temperature, in order to thermally dissolve the metal salt and melt the metal particles thus created.
According to the spray pyrolysis method, the metal powder thus obtained is spherical with excellent crystallization properties and with high dispersant characteristics. According to the spray pyrolysis method, for example, Ag powder can be formed with the maximum particle size of 1.7 μm and the minimum particle size of 0.5 μm using a solution containing AgNO3; Ag.Pd alloy powder is formed with particle sizes ranging from 2.5 μm (max) to 1.5 μm (min) by using a solution containing AgNO3 and Pd (NO3)2, and Au powder is formed with particle sizes ranging from 1.0 μm (max) to 0.5 μm (min) using a solution containing HAuCl4. Also, these powders are said to have excellent crystalline characteristics.
In this manner, metal powder with particle sizes ranging from 0.5 to 2.5 μm and excellent crystalline characteristics can be obtained. Metal powder with these properties is suitable as conductive paste.
However, the examples described above pertain to Ag, Ag.Pd alloy and Au, but not to metal powder, especially Fe powder, that is suitable for using the mixing and dispersing of magnetic powder.
Prior art teaches methods of manufacturing metal powder by the spray pyrolysis method, and suggests the possibility of manufacturing Fe powder or Fe alloy powder. However, we have not as yet seen an example of actually manufacturing Fe powder or Fe alloy powder. In other words, it can be said that metal powder that can be manufactured by the spray pyrolysis method had imposed considerable restrictions on the types of metal powder.
It is noted that Fe powder or Fe alloy powder can be manufactured from gaseous phase and solid phase as explained above. However, the particle size of metal particles formed by the gaseous phase manufacturing method is very small, and thus, unsuitable to be mixed with resin. Also, metal powder formed from the solid phase manufacturing method has poor particle distribution and the shape of the powder particles is not spherical because crushing machines are used.
Thus, magnetic metal powder, especially Fe or Fe alloy powder with properties suitable to be mixed with resin were unavailable from conventional metal powder manufacturing methods.